Clinical applications traditionally involve image data, which needs to be analyzed and interpreted. Based on interpretation of the image data, a physician can make a diagnosis and advise a treatment suitable for a patient. Proper interpretation of an image computed from the image data often requires measuring objects describing anatomical and pathological structures visualized in the image. To this end, the physician needs a tool which allows her/him to assess the diameter of a blood vessel or the size of a tumor, for example. In most applications, to measure an object, the user determines two points in a three-dimensional image data space, hereinafter referred to as 3D points, and the application is arranged to calculate the distance between these 3D points. A line segment connecting the determined 3D points may be displayed.
Unfortunately, it is not always possible to accurately determine two points on the surface of a measured structure such as a blood vessel, for example. This is because the surface, on which a point is to be selected, is typically perpendicular to the viewing plane. FIG. 1 illustrates the problem of the prior art method. The goal is to measure the diameter of the blood vessel 11. Two points, connected by a line segment 12, are selected on the two opposite edges in the rendered image of the blood vessel. The diameter of the vessel 11 is typically underestimated, as shown on the zoomed-in vessel segment 13.
To overcome this problem, in current applications, the user needs to make edges in the rendered image of the blood vessel clearly visible. This is achieved by positioning the blood vessel in the image to make the first edge visible and determining the first point. Then the user repositions the vessel to make the second edge visible and determines the second point. Alternatively, the user can zoom in the vessel, select the two points, and zoom out the vessel to its original position. Unfortunately, these operations require extra user interaction to measure the blood vessel. A further problem is that the drawn line segment may be not aligned perpendicularly to the vessel axis, as it should be.
To measure objects visualized in two-dimensional images, an on-screen caliper may be used. An implementation of such a caliper is offered on-line by Inico at http://www.iconico.com/caliper/index.aspx, retrieved Jul. 12, 2007. This caliper, however, obstructs the view of structures visualized in an image.